Reverse the iteration order for focus check.

Iterates the SparseArray of window decors in reverse order when
searching for focused task. This avoids an issue where the decor of a
closed task that has the isFocused flag set to true is found, preventing
fullscreen tasks from being dragged into desktop.

Bug: 296921167
Test: Manual; open a test app via studio, lock the screen, open the same
test app through studio again. Confirm drag to desktop works after doing
this.

Change-Id: I64ff0414e38109fb15905d37cf7cb7d7ec4ead5a